This invention relates to an acceleration sensor for detecting acceleration acting on a vehicle, and more particularly to an acceleration sensor of this kind which can be used, for example, in controlling a passive seat belt, one end of which is fixed to a retractor located on the floor of the vehicle compartment, and the other end can be moved forward and backward along a roof rail as the door is opened and closed, and in controlling a fuel pump. For example, the acceleration sensor can be used so that the other end of the seat belt may be prevented from moving forward along the roof rail even if the door is opened, or to stop the operation of the fuel pump, when acceleration caused by a crash of the vehicle is detected.
Conventionally, acceleration sensors have been proposed, e.g. by Japanese Patent Publication (Kokoku) No. 50-14345, U.S. Pat. No. 4,326,111, and Japanese Provisional Utility Model Publication (Kokai) No. 2-21563, in which an inertial mass is held in a predetermined position by at least one of gravity, the force of a spring, and a magnetic force, before a predetermined or larger magnitude of acceleration acts thereon, while once the predetermined or larger magnitude of acceleration acts thereon, the inertial mass is displaced from the predetermined position to actuate a switch.
However, the conventional acceleration sensors require a special holding mechanism to hold the inertial mass in the displaced position, which utilizes snap action of a spring. Therefore, the sensors have complicated constructions.